The fate of fixed nitrogen in Santa Barbara Basin sediments during seasonal anoxia

Abstract. Despite long-standing interests in the biogeochemistry of the Santa Barbara Basin (SBB), there are no direct rate measurements of different nitrogen transformation processes. We investigated benthic nitrogen cycling using in-situ incubations with ¹⁵NO₃^- addition and quantified the rates of total nitrate (NO₃^-) uptake, denitrification, anaerobic ammonia oxidation (anammox), N₂O production, and dissimilatory nitrate reduction to ammonia (DNRA). Denitrification was the dominant NO₃^- reduction process, while anammox contributed 0–27 % to total NO₃^- reduction. DNRA accounted for less than half of NO₃^- reduction except at the deepest station at the center of the SBB where NO₃^- concentration was lowest. NO₃^- availability and sediment total organic carbon content appeared to be two key controls on the relative importance of DNRA. The negative correlation between NO₃^- availability and the relative importance of DNRA suggests a negative feedback loop that potentially contributes to stabilizing the fixed N budget in the SBB. Nitrous oxide (N₂O) production as a fraction of total NO₃^- reduction ranged from 0.2 % to 1.5 %, which was higher than previous reports from nearby borderland basins. A large fraction of NO₃^- uptake was unaccounted for by NO₃^- reduction processes, suggesting that intracellular storage may play an important role. Our results indicate that the SBB acts as a strong sink for fixed nitrogen and potentially a net source of N₂O to the water column.